Computer crashing

Hi, my computer crashes without any warning mostly during gameplay or sometimes just looking at sites. I've had this problem for months and really need help to solve this. I had enough of this crashing, my computer is nearly completely useless because of this crashing and its only 1yr. old.
My PC: Windows XP
NVIDIA GeForce4 MX 440
Version 2002, Service Pack 1
Pentium(R) 4 CPU 2.66GHz
512 MB of RAM
I got this blue screen once with Technical information:
*** STOP: 0x0000007F (0x0000000D, 0x00000000, 0x00000000, 0x00000000)

Also, a few times of gotten this error when it crashes. When it starts to load back in the screen telling all of your computer stats it says "Hardware Monitor found an error. Please go to Power Setup menu." or something like that.